A motion transmission system in a bicycle comprises a chain extending between toothed wheels associated with the axle of the pedal cranks and with the hub of the rear wheel. When there is an assembly of toothed wheels comprising more than one toothed wheel at at least one of the axle of the pedal cranks and the hub of the rear wheel, and the motion transmission system is therefore provided with a gearshift, a front derailleur and/or a rear derailleur are provided for.
In order to increase the speed and precision with which gearshifting can be carried out, so-called electronic gearshifts, or more correctly electronically servo-assisted gearshifts, have become common.
In case of an electronically servo-assisted gearshift, each derailleur comprises a guide element—also called chain guide or cage or, in case of a rear derailleur, rocker arm—movable to displace the chain among the toothed wheels in order to change the gear ratio, and an electromechanical actuator to displace the chain guide. The actuator in turn typically comprises a motor, typically an electric motor, coupled with the chain guide through a linkage such as an articulated parallelogram, a rack system or a worm screw system, as well as a sensor or transducer of the position, speed, acceleration and/or direction of rotation, of the rotor or of any mobile part downstream of the rotor, down to the chain guide itself. It is worthwhile noting that slightly different terminology from that used in this context is also in use.
Control electronics changes the gear ratio automatically, for example based on one or more detected variables, such as the travel speed, the cadence of rotation of the pedal cranks, the torque applied to the pedal cranks, the slope of the travel terrain, the heart rate of the cyclist and similar, and/or the gear ratio is changed based on commands manually input by the cyclist through suitable control members, for example levers and/or buttons.
A device or unit for controlling the front derailleur and a device or unit for controlling the rear derailleur—or only one of the two in case of simpler gearshifts—are mounted so as to be easily maneuvered by the cyclist, normally on the handlebars, close to the handgrips thereof where the brake lever is also located for controlling the brake of the front and rear wheel, respectively. Control devices that allow driving both a derailleur in the two directions and a brake are commonly called integrated controls.
It is in general known to drive the derailleur of a bicycle gearshift with reference to values of a table of command values, each correlated to a position of the derailleur in which engagement of the chain with a specific toothed wheel occurs, or there is the engagement. In other words, the control electronics or controller uses the table of command values to obtain the value that a variable of the derailleur must take to position the chain in engagement with the desired toothed wheel. Such a value can be a differential value with respect to the adjacent toothed wheel, or it can be an absolute value with respect to a reference, for example with respect to a reference toothed wheel or to an end of stroke condition or a condition of lack of excitation of the motor.
From the point of view of magnitude, an actuator command value of the table of values can be, for example, the distance traveled by a mobile point taken as reference on the derailleur, the number of steps or revolutions that the motor should be made to perform, a length of excitation time of the motor, the value of a supply voltage of a motor having an excursion proportional to the voltage, or furthermore it can be the value emitted by the sensor or transducer associated with the motor, a numerical value stored in a register and representative of one of the aforementioned quantities, etc.
In particular, the motors of actuators can be driven for a number of steps or for a length of excitation time or with a voltage that are appropriate for each upward or downward gearshifting and then stopped automatically, while the sensors are used to provide a feedback signal to the control electronics so that it can possibly take care of actuating the motors of the actuators again in case the intended position has not been reached, namely in case the aforementioned variable of the derailleur has not taken the table value. This may, for example, be due to the fact that the resistance torque offered by the derailleur, which is to a certain extent dependent on how the cyclist is pedaling, was too high, greater than the maximum torque able to be delivered by the motors through the linkage.
The values of said table of command values are nominal values, set in the factory, which take the number of toothed wheels in the derailleur (front or rear) and the respective thicknesses and pitches into account. Typically, such nominal values provide that, in the absence of the actuator driving signal, namely with command value at zero, the chain is in engagement with the toothed wheel having the smallest diameter, although, as can be seen from the aforementioned examples, this condition is not necessary.
It is also known to drive the electric motor of the derailleur through a voltage controlled over time according to a certain predetermined profile.
The predetermined profile of the drive voltage of the motor is feedback controlled based on the residual distance from the destination position. For this purpose, the derailleur typically comprises the aforementioned position sensor, suitable for detecting the instantaneous position of the drive shaft or of a shaft or other component set into rotation or more in general in motion by the drive shaft. The residual distance is calculated as the difference between the destination position read from the table of command values and the current position detected by the sensor, which are assumed to be homogeneously expressed.
In order to obtain an adequately smooth movement, the profile of the drive voltage is typically an inverse quadratic function of the residual distance. Therefore, as the destination position is approached, the drive voltage of the motor decreases.
It is also known to monitor the time elapsed since the starting of the electric motor for the derailleur displacement, and to interrupt the power supply to the electric motor in any case once a time limit has been exceeded, so as to protect the motor itself and the components in case some jamming or other mechanical impediment causes an excessive resistance that the motor is unable to overcome, and therefore prevents the desired displacement.
The technical problem at the basis of the present invention consists of improving the control of the motor of a derailleur of a bicycle electronic gearshift.